Optimising variadic functions seems fairly low value but given they're usually a different calling convention anyway passing an extra integer is probably harmless.
From what I understand, VSI's C compiler for OpenVMS/x86-64 actually gives you a choice of two different variadic calling conventions - something very close to standard SysV ABI x86-64 (which doesn't pass parameter count and type info), and the OpenVMS extension which adds that. So, while I doubt one extra "mov rax, CONSTANT" is going to be noticed (variadic function calls are rarely found on performance-critical code paths), if a function doesn't need va_count(), it can turn that off.
From what I understand, their x86 C and C++ compilers are a patched version of clang/LLVM – they've said they planned to upstream their changes, I don't think it has happened yet, but I hope it does. (Their other compilers – COBOL, Fortran, Pascal, BASIC, BLISS, MACRO – are also using LLVM as backend on x86-64, but with their legacy frontends.)
From what I understand, their x86 C and C++ compilers are a patched version of clang/LLVM – they've said they planned to upstream their changes, I don't think it has happened yet, but I hope it does. (Their other compilers – COBOL, Fortran, Pascal, BASIC, BLISS, MACRO – are also using LLVM as backend on x86-64, but with their legacy frontends.)